An electric power steering apparatus, that applies an assist load to a steering apparatus of an automobile or a vehicle by means of rotational force of a motor, applies an assist load to a steering shaft or a rack shaft by transmitting a driving power of the motor using a transmission mechanism such as gears or a belt via a speed reduction device. In order to accurately generate an assist torque (steering assist torque), such a conventional electric power steering apparatus carries out a feedback control of motor currents. The feedback control adjusts a motor applying voltage so that a difference between a current command value and a detected motor current value becomes small or zero, and the adjustment of the motor applying voltage is generally made by adjusting a duty ratio of a pulse width modulation (PWM) control.
A general constitution of the electric power steering apparatus is explained with reference to FIG. 1. A column shaft 2 of a steering wheel 1 is connected to a tie rod 6 via reduction gears 3, universal joints 4a and 4b and a pinion rack mechanism 5. The column shaft 2 is provided with a torque sensor 10 that detects a steering torque of the steering wheel 1, and a motor 20 that assists a steering power of the steering wheel 1 is connected to the column shaft 2 via the reduction gears 3. An electric power is supplied from a battery 14 via an ignition key 11 to a control unit 100 that controls the power steering apparatus, and the control unit 100 operates a steering assist command value I of an assist command based on a steering torque signal Tr detected by the torque sensor 10 and a vehicle speed signal Ve1 detected by a vehicle speed sensor 12 so as to control an electric current to be supplied to the motor 20 based on the operated steering assist command value I.
In such an electric power steering apparatus, as disclosed in Japanese Patent Application Laid-open No. 8-290778 A, for example, conventionally a robust stabilizing compensator in the control unit 100 simultaneously designs a stability of a system and sensitivity characteristics of a road information and a disturbance information.
In the conventional control apparatus, however, since a reaction force at a time of a steering near a steering neutral point is small, it is difficult to accurately transmit the road information to a driver due to an influence of friction. Further, in the conventional electric power steering apparatus, it is difficult to make a hysteresis characteristic between a steering angle and a steering power equivalent to a characteristics of a hydraulic power steering.
An apparatus which solves such problems is disclosed in Japanese Patent Application Laid-open No. 2002-369565 A.
An outline of the apparatus disclosed in Japanese Patent Application Laid-open No. 2002-369565 A is explained with reference to FIG. 2 related with FIG. 1. The motor 20 that generates an assist steering power of the steering apparatus is driven by a motor driving section 21, the motor driving section 21 is controlled by the control unit 100 shown by alternate long and two short dashes line, and the steering torque signal Tr from the torque sensor and the vehicle speed signal Ve1 from the vehicle speed detecting system are inputted to the control unit 100. In the motor 20, a motor inter-terminal voltage Vm and a motor current value i are measured so as to be outputted.
The control unit 100 is composed of a torque-system control section 110 shown by a broken line that makes a control using the steering torque signal Tr, and a motor-system control section 120 shown by alternate long two short dashes line that makes a control relating to the driving of the motor 20. The torque-system control section 110 is composed of an assist amount operating section 111, a differentiation controller 112, a yaw-rate convergence control section 113, a robust stabilization compensating section 114 and an SAT estimating feedback section 115. The torque-system control section 110 has adders 116A and 116B and a subtracter 116C. Further, the motor-system control section 120 is composed of a compensator 121, a disturbance estimator 122, a motor angular speed estimating section 123, a motor angular acceleration estimating section (differentiator) 124 and a motor characteristic compensating section 125, and has adders 126A and 126B.
The steering torque signal Tr is inputted to the assist amount operating section 111, the differentiation controller 112, the yaw rate convergence control section 113 and the SAT estimating feedback section 115, and at this time, the vehicle speed signal Ve1 is inputted to them as a parameter. The assist amount operating section 111 operates an assist torque amount based on the steering torque signal Tr, the yaw-rate convergence control section 113 inputs the steering torque signal Tr and an estimated value co of the motor angular speed and applies the brake to a swing operation of the steering wheel in order to improve the yaw convergence characteristics of the vehicle. Further, the differentiation controller 112 improves a responsiveness of control near the steering neutral point and realizes a smooth steering. The SAT estimating feedback section 115 inputs the steering torque signal Tr, a signal which is obtained by adding an output from the differentiation controller 112 to an output from the assist amount operating section 111 by means of the adder 116A, the angular speed estimated value ω which is estimated by the motor angular speed estimating section 123 and the angular acceleration estimated value *ω from the motor angular acceleration estimating section 124 therein so as to estimate the SAT, and executes a signal process on the estimated SAT using a feedback filter, so as to supplies the road information suitable for the steering wheel as a reaction force.
A signal, which is obtained by adding the output from the yaw rate convergence control section 113 to the signal obtained by adding the output from the differentiation controller 112 to the output from the assist amount operating section 111 by means of the adder 116B, is inputted as an assist amount AQ to the robust stabilization compensating section 114. The robust stabilization compensating section 114 is, for example, a compensating section disclosed in Japanese Patent Application Laid-open No. 8-290778 A, and removes a peak value at a resonance frequency of a resonance system composed of an inertial element and a spring element included in the detected torque, and compensates a phase shift of the resonance frequency that disturbs the responsiveness and the stability of the control system. The subtracter 116C subtracts the output of the SAT estimating feedback section 115 from the output of the robust stabilization compensating section 114 so as to obtain an assist amount Ia for enabling the road information to be transmitted as the reaction force to the steering wheel.
Further, the motor angular speed estimating section 123 estimates the motor angular speed a based on the motor inter-terminal voltage Vm and the motor current value i, and the motor angular speed ω is inputted to the motor angular acceleration estimating section 124, the yaw rate convergence control section 113 and the SAT estimating feedback section 115. The motor angular acceleration estimating section 124 estimates a motor angular acceleration based on the inputted motor angular speed ω, and the estimated motor angular acceleration *ω is inputted to the motor characteristic compensating section 125. The adder 126A adds an assist amount Ia, which is obtained by subtracting the output of the SAT estimating feedback section 115 from the output of the robust stabilization compensating section 114, to the output Ic of the motor characteristic compensating section 125, and the added signal is inputted as a current command value Ir to the compensator 121 as the differentiation compensator or the like. A signal, which is obtained by adding the output of the disturbance estimator 122 to the current command value Ira compensated by the compensator 121 by means of the adder 126B, is inputted to the motor driving section 21 and the disturbance estimator 122. The disturbance estimator 122 is a device disclosed in Japanese Patent Application Laid-open No. 8-310417 A, and can maintain a desired motor control characteristics in an output basis of the control system based on a signal as a control target of the motor output which is obtained by adding the output of the disturbance estimator 122 to the current command value Ira compensated by the compensator 121 and the motor current value i, so that the stability of the control system is prevented from being lost.
A state of the torque generated between the road surface and the steering is explained with reference to FIG. 3. When a driver steers the steering wheel 1 so that a steering torque Th is generated, and the motor 20 generates an assist torque Tm according to the steering torque Th. As a result, wheels are steered, and an SAT is generated as the reaction force. At this time, an inertial J and a friction (static friction) Fr of the motor 20 generate a torque as resistance of the steering of the steering wheel 1. When the balance of these forces is considered, the following motion equation (1) is obtained:J·*ω+Fr·sign(ω)+SAT=Tm+Th  (1)When the above equation (1) whose inertial value is zero is Laplace-transformed and is solved for the SAT, the following equation (2) is obtained:SAT(s)=Tm(s)+Th(s)−J·*ω(s)−Fr·sign(ω(s))  (2)As is clear from the equation (2), when the inertial J and the static friction Fr of the motor 20 are previously obtained as constant, the SAT can be estimated based on the motor rotational angular speed ω, the rotational angular acceleration *ω, the steering assist force and a steering signal. For this reason, the steering torque signal Tr, the angular speed ω, the angular acceleration *ω, and the output of the assist amount operating section 111 are inputted to the SAT estimating feedback section 115.
When the SAT information estimated by the SAT estimating feedback section 115 is directly feedbacked, the steering becomes too heavy, and thus steering feeling cannot be improved. For this reason, as shown in FIG. 4, the estimated value of the SAT is subject to the signal process by using a feedback filter 115A having a vehicle speed sensitive gain and a frequency characteristic, and only information which is sufficiently necessary for improving the steering feeling is feedbacked. The feedback filter to be used here has a Q filter (phase delay) 115B having a gain for reducing the estimated SAT to the sufficiently necessary value as a static characteristic gain and a gain section 115C which is sensitive to the vehicle speed Ve1 as shown in FIG. 5, and when the importance of the road information such as a stationary steering and a low-speed traveling is comparatively low, the road information to be feedbacked is reduced.
In the above apparatus disclosed in Japanese Patent Laid-open No. 2002-369565 A, since the SAT functions as disturbance for the electric power steering, the SAT is estimated by a disturbance observer constitution. The phase delay filter (Q filter) is used for the feedback because the estimated SAT as the disturbance is prevented from being diffused in the disturbance observer. The estimated SAT value, however, naturally becomes a value after passing the Q filter (phase delay), and thus a delay is generated in the steering transmission system.
In the apparatus disclosed in Japanese Patent Laid-open No. 2002-369565 A, the SAT estimating feedback is also constituted so that a frequency band where disturbance which is desired to be suppressed is present is compatible with a frequency band where disturbance which is desired to be transmitted is present, but does not have a function for positively canceling the disturbance which is desired to be suppressed.
On the other hand, in vehicles, brake judder and shimmy which make uncomfortable to vehicle occupants occur at a time of a normal braking and a stationary traveling. The brake judder means a floor/pedal vibration which is generated at the time of braking of vehicles, and occasionally causes the vibration to a steering rotational direction. The source of the vibration is a braking torque variation generated due to DTV (Disk Thickness Variation) of a brake disc, and the braking torque variation has a primary component and a high-order component of a rotation of wheels. This is amplified by resonance before or after suspension or the like, the amplified brake judder is transmitted to a vehicle body or a steering system so as to become the floor/pedal vibration or the steering vibration. Further, the shimmy is vibration which is generated in the steering rotational direction when a vehicle travels, and its source is unbalance and non-unifoiniity of rotating portions of tire wheels. The shimmy is amplified by suspension resonance and becomes the vibration in the steering rotational direction via the steering system.
Such the brake judder and the shimmy are not taken into consideration at all in the apparatus of Japanese Patent Laid-open No. 2002-369565 A. Japanese Patent Application Laid-open No. 2002-145075 A and Japanese Patent Application Laid-open No. 2002-161969 A disclose apparatus that attenuate vibration due to the brake judder and the shimmy, but they mechanically cope with the brake judder and the shimmy, thereby arising problems such that the cost increases and fine suppression, such as vehicle speed sensitivity, cannot be performed.
Further, when the inertial and the friction of the steering system are large, the vibration due to the brake judder is not transmitted to the steering wheel, but it is desirable that the inertial and the friction of the steering system are as small as possible in order to obtain a satisfactory steering feeling and a stability of the vehicle.
Still further, the control unit 100 could be also described as FIG. 6. The steering torque value T detected by means of the torque sensor 10 to be inputted and the vehicle speed V from the vehicle speed sensor 12 are inputted to a current command value operating section 31 for operating a current command value Iref. The current command value operating section 31 determines the current command value Iref, which is a control target value of a current supplied to the motor 20, on the basis of the inputted steering torque value T and the inputted vehicle speed V. The current command value Iref is inputted to a subtracter 32 so that a deviation I (Iref-Im) from a motor current value Im, which is feedbacked, would be operated. The deviation is inputted to a PI-controlling section 33 for improving a specific property in a steering operation. A steering assist command value Vref improved in specific property in the PI-controlling section 33 is inputted to a PWM-controlling section 34 to PWM-drive the motor 20 through an inverter 35 provided as a driving part. The current value Im of the motor 20 is detected by means of a motor current detector 36 to be feedbacked to the subtracter 32. The inverter 35 includes FETs used as driving devices and comprises a bridge circuit of the FETs.
Such an electric power steering apparatus has a self-aligning torque (SAT) function. That is to say, wheels of a vehicle automatically returns to a neutral position when a driver reduces force for rotating a steering wheel or makes the force zero (namely, a hand-losing state of losing his or her hold of the steering wheel) in a process of returning to a straight run after a changing steering. Such the SAT for returning to the neutral position becomes larger, the faster the vehicle speed is. In the electric power steering apparatus, the wheels move in a direction to the neutral position (leftward) in accordance with the SAT function when the wheels are first changed rightward in the steering, for example. The steering torque should be zero since the force for rotating the steering wheel by a driver is zero. Accordingly, the steering torque value detected by the torque sensor also becomes zero, a current is not applied to the motor, the steering assist power is not generated, and therefore, the wheels rotate leftward as they are connected to the steering apparatus. The steering wheel, of course, also rotates leftward.
The conventional electric power steering apparatus, however, has a problem that a frictional power or the like, which corresponds to the friction of the rotor of the motor and the frictions of the vehicle and the steering system, deteriorates the returning of the steering wheel after the changing steering in a low-speed run and deteriorates an on-center feeling such as a steering wheel returning feeling and a friction feeling in a high-speed run. That is to say, in a process that a driver rotates the steering wheel and changes the steering at a low speed to return to the straight run, the electric power steering apparatus is so worse in returning of the steering wheel than a manual steering apparatus or a hydraulic power steering apparatus that the driver must rotate the steering wheel again to a direction of a straight run in an extreme instance. Further, in a process to return to the straight run after the changing steering for the purpose of changing lanes or correcting a direction in the high-speed run (especially in a process of returning in a hand-losing state without holding the steering wheel), the wheels cannot completely return to the center position (the neutral position) even when the SAT function lets the wheels return to the neutral position. In an extreme case, the driver must rotate the steering wheel in a direction of the straight run once more.
As a solution method of such a problem, proposed has been a power steering apparatus in which a road surface reaction force torque detector is used as a steering wheel returning correcting section for correcting or removing the friction, a steering wheel angle detector for detecting an angle of rotation of the steering wheel is provided as the road surface reaction force torque detector, a steering wheel returning correction quantity is obtained on the basis of a steering wheel angle signal outputted from the steering wheel angle detector and the correction quantity is used for adjusting imbalance in the friction, which is generated in accordance with a rotation or a direction of movement of the steering wheel and the steering system, in order to even a left-and-right difference of a returning property of the steering wheel.
Similarly, also provided has been a power steering apparatus in which a road surface reaction force torque detector as the steering wheel returning correcting section for correcting or removing the friction is formed from a road surface reaction force torque estimating means, the road surface reaction force torque estimating means is used for generating a road surface reaction force torque estimation signal by performing a low pass filter (LPF) or a delay filter process for a value obtained by adding the motor torque operated in terms of a steering shaft on the basis of an output of a current detector for detecting a current of the motor to an output of the steering torque detector and by subtracting the motor inertia torque in terms of the steering shaft, the steering wheel returning correction quantity is obtained on the basis of the road surface reaction force torque estimation signal and the correction quantity is used for adjusting imbalance in the friction, which is generated in accordance with a rotation or a direction of movement of the steering wheel and the steering system, in order to even a left-and-right difference of a returning property of the steering wheel.
In such conventional examples, there is a shortcoming that the steering power is too heavy in turning the steering wheel when the friction of the vehicle is large although an effect of improving returning of the steering wheel in losing driver's hold of the steering wheel can be expected in the case of a low speed. Generally, the manual steering power without the power assist in turning the steering wheel during a turn at a bend becomes a value obtained by adding the reaction force from the road surface to the friction of the steering system, and thus, is heavier than the road surface reaction force by the friction. To the contrary, the manual steering power in returning the steering wheel becomes a value obtained by subtracting the friction of the steering system from the road surface reaction force, the value being lighter than the road surface reaction force by the friction. This mainly causes deterioration in returning of the steering wheel. Accordingly, especially in the case of an automobile having a large value of friction of the vehicle and the steering system, the steering wheel returning correction quantity for overcoming the friction to return the steering wheel tends to increase. This causes a drawback that performing the correction in turning and returning with a same coefficient on the basis of the road surface reaction force results in excessively heavy power in returning and turning the steering wheel during a turn at a bend in a low-speed run, which especially requires good returning of the steering wheel from a position at a large steering angle, and thereby, results in great imbalance.
In order to solve the problems, proposed is the control apparatus for the electric steering system disclosed in Japanese Patent Application Laid-open No. 2002-29441 A. The control apparatus is the electric power steering apparatus provided with a motor interposed in a steering torque transmission mechanism from a steering wheel to wheels to generate a torque for assisting the steering torque by a driver, the electric power steering apparatus comprising a steering wheel return correcting section for correcting the friction of a rotor of the motor and the friction of a vehicle and a steering system and a steering state discriminative compensating section for discriminating the turning and returning of the steering wheel, wherein the correction quantity of the steering wheel return correcting section is set to different values according to the turning and returning directions of the steering wheel.
The control apparatus disclosed in Japanese Patent Application Laid-open No. 2002-29441 A, however, only puts a bad returning of the steering wheel during the low speed rotation of the steering wheel in question, but does not take the SAT into account. Accordingly, the road surface reaction force of a vehicle is so strong that the steering wheel suddenly returns excessively in some cases. This causes a strong requirement for the solution.
Recently, a vehicle on which an electric power steering is mounted has been further increased in size. This causes a problem that a good steering feeling (especially turning and returning of a steering wheel) cannot be achieved in controlling a conventional small-sized vehicle.
The present invention is devised in order to solve the above problems, and an object is to provide a control apparatus for an electric power steering apparatus that executes a signal process on road information, disturbance or the like in a frequency area without delay so as to be capable of obtaining a safe and comfortable steering performance which provides an easy tuning, and a brake judder and shimmy suppression.
Another object of the present invention is to provide a control apparatus for an electric power steering apparatus with a high performance and applicable in both of the cases that the returning of the steering wheel is bad and that the steering wheel excessively returns by taking a self-aligning torque (SAT) into account.